1. Field of the Invention
The present invention relates to systems for producing multiple cyclic output signals having predetermined time relations to each other and more particularly to such systems wherein the times of occurrence of the signals during the cycles, i.e., their "phase" relationships, can be selectively shifted.
One typical environment for selective phase shifting of multiple signal pulses is in the production of the image from a single television camera on multiple, coordinated television displays so that each display screen exhibits a portion of the camera image.
In such systems the camera operation is generally controlled by two or more command signals, usually called drive pulses, which are cyclic, may have different durations and occur at different respective times in the cycle interval. Likewise each display may be controlled by multiple cyclic drive pulses which have different durations and occur at different respective times in the cycle.
Because a predetermined part of the camera image is depicted by each display, operation of each display must be closely and accurately coordinated with the other displays and with the camera itself. As an example of the criticality of the coordination required, in many circumstances the camera is located remotely from the displays and the very slight delay attendant the passage of electrical signals through transmission cables to the displays is sufficiently long to adversely affect the image quality. This delay in transmission through the cable is called "propagation delay."
In order to accommodate the display operation to the camera, the camera operating drive pulses must be coordinated with display drive pulses in accordance with the extent of propagation delay being presently experienced. This requires that the system user have the ability to change the phase relationship between the camera and display drive pulses. As noted, there may be a significant number of display and camera drive pulses and consequently each display drive pulse must be shifted in time relative to the appropriate camera drive pulses in order to avoid problems resulting from the propagation delay.
If multiple cameras are used and the propagation delay from one camera is different from that of another, the display drive pulses should be simultaneously, and virtually instantaneously, adjusted if coherent successive images from different cameras are to be displayed.
In addition to shifting the phase of multiple display drive pulses relative to a given camera drive phase it is sometimes necessary, or at least desirable, to simultaneously alter the durations of the display drive pulses. This is done, for example, when the sweep frequency of the display units is to be altered.
Another situation in which display unit drive signals must be altered with speed and accuracy is where a television camera image which is substantially smaller than the screen area of the display unit, or units, moves around relative to the display unit, or units.
2. The Prior Art
Techniques for advancing or delaying cyclical signal pulses relative to each other have been used in the past. Analog phase shifters have been proposed for use in advancing or delaying signal pulses relative to each other. In one such proposal a first drive pulse initiated operation of a voltage ramp generator whose output was connected to the input of an analog comparator. The other comparator input was connected to a reference voltage so that when the ramp generator output equaled the reference voltage the comparator changed stated thereby creating an output signal. The reference level was adjustable to alter the extent of delay of the comparator output.
The components of analog systems were accurate and stable only over narrow operating condition ranges and therefore the extent of the advance or delay was limited by the quality of the system components. The components also tended to pick up electrical "noise" which affected the extent of the delay unsatisfactorily. Thirdly it was found that the ramp generators cycled at frequencies which were too low for some applications.
Many of the shortcomings of the analog systems were found to be avoidable by the use of digital electronic systems. One such digital system is disclosed by U.S. Pat. No. 3,833,854 issued to Robert W. Schonover on Sept. 3, 1974. The Schonover patent discloses a clock driven counter comprising a number of flip-flops connected to form an asynchronous counter whose outputs are connected to an AND gate for controlling operation of a second counter. Both counters are run by a common clock and loading the "set" inputs on the second counter results in a "delay" of the second counter's output relative to the first counter's output.
Although the Schonover device overcomes some disadvantages of the analog phase shifting systems, that system did not enable simultaneous phase shifting of multiple signals. In some circumstances when multiple display drive pulses were shifted relative to a camera drive pulse it has been possible to inadvertantly extend or shorten display drive pulse durations sufficiently that damage to deflection signal amplifier circuitry in the displays tended to occur. Moreover the Schonover circuitry was not sufficiently fast acting to enable phase shifting within a single cycle. The Schonover system limited to one per cycle the number of delayed or advanced pulses which could be accommodated. If, for example, a three bit counter were used in the Schonover system a different output pulse drive from the second counter could be produced only after eight clock pulses were counted.
Thus, while the Schonover approach avoided many of the serious shortcomings of analog phases shifters it did not provide a means for instantaneous or simultaneous phase shifting of cyclic drive pulses relative to each other.